Kazimierz inherited a kingdom weakened by war and made it prosperous and wealthy. He reformed the Polish army and doubled the size of the kingdom. He reformed the judicial system and introduced a legal code, gaining the title "the Polish Justinian". Kazimierz built extensively and founded the University of Kraków, the oldest Polish university. He also confirmed privileges and protections previously granted to Jews and encouraged them to settle in Poland in great numbers.

Kazimierz left no lawful male heir to his throne, producing only daughters. When Kazimierz died in 1370 from an injury received while hunting, his nephew, King Louis I of Hungary, succeeded him as king of Poland in personal union with Hungary.

Poland (red) at the end of the reign of Kazimierz III (1370); Silesia (yellow) had been lost, but the Kingdom was expanding to the east

When Kazimierz attained the throne in 1333, his position was in danger, as his neighbours did not recognise his title and instead called him "king of Kraków". The kingdom was depopulated and exhausted by war, and the economy was ruined. In 1335, in the Treaty of Trentschin, Casimir was forced to relinquish his claims to Silesia "in perpetuity".

Kazimierz rebuilt and his kingdom became prosperous and wealthy, with great prospects for the future. He waged many victorious wars and doubled the size of the kingdom, mostly through addition of lands in modern-day Ukraine (then called the Duchy of Halych). Kazimierz built extensively during his reign, including Wawel Castle and Orle Gniazda, and he reformed the Polish army

At the Sejm in Wiślica, on 11 March 1347, Kazimierz introduced reforms to the Polish judicial system and sanctioned civil and criminal codes for Great and Lesser Poland, earning the title "the Polish Justinian". He founded the University of Kraków, the oldest Polish University, and he organized a meeting of kings in Kraków in 1364 at which he exhibited the wealth of the Polish kingdom.[citation needed] Kazimierz is the only king in Polish history to both receive and retain the title of "Great" (Bolesław I Chrobry is also called "Great", but more commonly "Valiant").

In 1355, in Buda, Kazimierz designated his nephew Louis I of Hungary as his successor should he produce no male heir, as his father had with Charles I of Hungary to gain his help against Bohemia. In exchange Kazimierz gained Hungarian favourable attitude, needed in disputes with the hostile Teutonic Order and Kingdom of Bohemia. Kazimierz at the time was still in his early years and having a son did not seem to be a problem (he already had a few bastard children).

Kazimierz left no legal son, however, bearing five daughters instead. He tried to adopt his grandson, Casimir IV, Duke of Pomerania, in his last will. The child had been born to his second daughter, Elisabeth, Duchess of Pomerania, in 1351. This part of the testament was invalidated by Louis I of Hungary, however, who had traveled to Kraków quickly after Kazimierz died and bribed the nobles with future privileges. Kazimierz III had a son-in-law, Louis VI of Bavaria, Margrave and Prince-elector of Brandenburg, who was considered a possible successor, but he was deemed ineligible as his wife, Kazimierz's daughter Cunigunde, had died in 1357 without issue.

Thus King Louis I of Hungary became successor in Poland. Louis was proclaimed king upon Kazimierz's death in 1370, though Kazimierz's sister Elisabeth (Louis's mother) held much of the real power until her death in 1380.[citation needed]

Casimir was facetiously named "the Peasants' King". He introduced the codes of law of Greater and Lesser Poland as an attempt to end the overwhelming superiority of the nobility. During his reign all three major classes — the nobility, priesthood, and bourgeoisie — were more or less counterbalanced, allowing Casimir to strengthen his monarchic position. He was known for siding with the weak when the law did not protect them from nobles and clergymen. He reportedly even supported a peasant whose house had been demolished by his own mistress, after she had ordered it to be pulled down because it disturbed her enjoyment of the beautiful landscape. {citation needed}

Casimir was favorably disposed toward Jews living in Poland. On 9 October 1334, he confirmed the privileges granted to Jews in 1264 by Bolesław V the Chaste. Under penalty of death, he prohibited the kidnapping of Jewish children for the purpose of enforced Christianbaptism, and he inflicted heavy punishment for the desecration of Jewish cemeteries. While Jews had lived in Poland since before his reign, Casimir allowed them to settle in Poland in great numbers and protected them as people of the king.[2]

Casimir III was born in Kowal, and he married four times. Casimir first married Anna, or Aldona Ona, the daughter of Grand Duke Gediminas of Lithuania. The marriage produced two daughters, Cunigunde (d. 1357), who was married to Louis VI the Roman, the son of Louis IV, Holy Roman Emperor, and Elisabeth, who was married to Duke Bogislaus V of Pomerania. Aldona died in 1339, and Casimir then married Adelaide of Hesse. He divorced Adelaide in 1356, married Christina, divorced her, and while Adelaide and possibly Christina were still alive (ca. 1365), he married Hedwig of Głogów and Sagan. He had three daughters by his fourth wife, and they were still very young when he died, and regarded as of dubious legitimacy because of Casimir's bigamy.

Casimir effectively divorced Adelaide and married his mistress Christina Rokiczana, the widow of Miklusz Rokiczani, a wealthy merchant. Her own origins are unknown. Following the death of her first husband she had entered the court of Bohemia in Prague as a lady-in-waiting. Casimir brought her with him from Prague and convinced the abbot of the Benedictineabbey of Tyniec to marry them. The marriage was held in a secret ceremony but soon became known. Queen Adelaide renounced it as bigamous and returned to Hesse without permission. Casimir continued living with Christine despite complaints by Pope Innocent VI on behalf of Queen Adelaide. The marriage lasted until 1363–64 when Casimir again declared himself divorced. They had no children.[citation needed]

With Adelheid still alive and Christina possibly surviving, the marriage to Hedwig was also considered bigamous. The legitimacy of the three last daughters was disputed. Casimir managed to have Anne and Cunigunde legitimated by Pope Urban V on 5 December 1369. Hedwig the younger was legitimated by Pope Gregory XI on 11 October 1371.[citation needed]

^"In Poland, a Jewish Revival Thrives—Minus Jews". New York Times. 12 July 2007. Probably about 70 percent of the world's European Jews, or Ashkenazi, can trace their ancestry to Poland — thanks to a 14th-century king, Casimir III, the Great, who drew Jewish settlers from across Europe with his vow to protect them as "people of the king",|access-date= requires |url= (help)

^Robert Frost, The Oxford History of Poland-Lithuania:Vol I, The Making of the Polish-Lithuanian Union, 1385-1569, (Oxford University Press, 2015), 28.